Magento: Add to cart button on upsell products in product detail page

To add a Add to Cart button on upsell products in product detail pages,

first open the upsell.phtml file located in app/design/frontend/your_package/your_theme/template/catalog/product/list/upsell.phtml

Then add a new form with Add to Cart button & quantity input box inside the loop that generate upsell products.

<form action="<?php echo $this->getAddToCartUrl($_link) ?>" method="post" id="product_addtocart_form_<?php echo $_link->getId()?>"<?php if($_link->getOptions()): ?> enctype="multipart/form-data"<?php endif; ?>>

<?php if(!$_link->isGrouped()): ?>
<input type="text" name="qty" id="qty" maxlength="12" value="<?php echo ($this->getMinimalQty($_link)?$this->getMinimalQty($_link):1) ?>" />
<label for="qty"><?php echo $this->__('Qty') ?>:</label>
<?php endif; ?>
<button type="button" onclick="this.form.submit()"><span><span><span><?php echo $this->__('Add to Cart') ?></span></span></span></button>

</form>